The present invention relates to a spectrophotometer, and, more particularly, to a spectrophotometer of a type that uses a motor to rotate a dispersive element for scanning over a range of wavelengths or setting a certain wavelength.
A sine bar has previously been used as a mechanism for performing scanning over a range of wavelengths on spectrophotometers equipped with a diffraction grating. In this mechanism, a nut is moved by means of a feed screw to push the sine bar projecting from the axis of the grating. Although the amount of rotation of the feed screw has a linear relationship with the wavelength, the use of a feed screw prevents fast feeding over a wavelength range. The recent advances in computer technology have made it relatively easy to convert the amount of rotation of a pulse motor quickly to a value of a desired function. Under this circumstance, the advantage of linear relationship between the amount of rotation of a feed screw and wavelength has gradually lost its appeal, and the difficulty involved in fast feeding over a wavelength range has become a major concern to the users of spectrophotometers adopting the sine bar mechanism. On the other hand, spectrophotometers of a type that allows a dispersive element to be directly rotated via a speed reducing mechanism have gained increasing popularity for several reasons including simple construction and low cost.
In this type of spectrophotometer which causes the dispersive element to be directly rotated via a speed reducing mechanism, the amount of rotation of the motor does not have a linear relationship with the wavelength even if a grating is used as the dispersive element. If a grating is used as the dispersive element, the relationship between the angle of rotation, .theta., of the grating from a reference position and wavelength .lambda. of output light is expressed by: ##EQU1## where d is the grating constant of the grating and n is the order of diffraction. As shown in FIG. 4, the angle .phi. is one half the angle formed between incident light on the grating G and diffracted light, that is, one half the angle subtended by the entrance and exit slits in the monochromator with respect to the center of the grating. Given equation (1) which describes the relationship between the angle of rotation of the grating and wavelength, the spectrophotometer of the type under consideration (which allows the dispersive element to be directly rotated via a speed reducing mechanism) converts the amount of rotation, x, of the driving motor to a corresponding wavelength using a ROM stored with a table that correlates the amount of x to wavelength by the following equation: EQU .lambda.=K.cndot.sinpx (.theta.=px). (2)
In equation (2), p is a constant determined by the speed reducing mechanism, but K includes not only the grating constant of the grating [see equation (1)] but also quantities associated with the positions of optical elements such as the entrance and exit slits in the monochromator. These quantities contain errors that will occur during the working and assembly of monochromator components and hence will differ slightly among individual units of spectrophotometer even if they are fabricated of the same design. Furthermore, it is difficult from a practical viewpoint to equip individual units of the device with a ROM stored with a table that was constructed by actual measurements of K in accordance with equation (2). Instead, the following procedure is usually taken: a ROM is preliminarily provided that is stored with a plurality of tables constructed for several values of K according to equation (2), and is mounted on each of the fabricated spectrophotometers; prior to shipment from the factory, a calibration test is conducted for each unit of the device to select an optimum table from those stored in the ROM; and when using a particular unit, the amount of rotation of the drive motor is converted to a corresponding wavelength using the selected table.
As described above, the spectrophotometer of the type that allows a dispersive element to be directly rotated with a motor via a speed reducing mechanism uses a conversion table to convert the amount of motor rotation to a corresponding value of wavelength. Ideally, each unit of the device must be equipped with a different conversion table due to the limited precision of working of monochromator components but, in practice, an optimum table is selected from among several prepared tables by a calibration test and used for subsequent spectrophotometric measurements. For this purpose, it is desirable to prepare the largest possible number of tables with slightly different contents. However, the number of tables that can be prepared is limited by the capacity of ROM and it sometimes occurs that an optimum table is not available for a certain spectrophotometer and that the value intermediate between two values in two different tables has to be used as the best one. With another device, the relationship between the amount of motor rotation and wavelength value may not fit any of the prepared tables. However, there has been no simple prior art method that is capable of dealing with these problems.